With increasing concern for the environment in recent years, electric and hybrid cars, which are wholly or partially driven by a motor, are growing in popularity. In these cars (hereinafter, vehicles), the electric power of the motor is supplied from a battery. Such batteries, however, are likely to change their characteristics or to be degraded while being rapidly charged or discharged with a large current. To avoid this, restrictions are imposed on the current supplied to the motor when the driver tries to accelerate too fast. This, however, sometimes makes acceleration insufficient.
To overcome this problem, there have been proposed vehicles having both a battery and capacitors with rapid charging-discharging characteristics. In such vehicles, the motor is supplied with electric power from both the battery and the capacitors when the driver tries to accelerate too fast. As a result, the vehicle can be accelerated faster than in the case of having a battery only.
To obtain a voltage high enough to drive a motor from capacitors, assuming that the voltage is about 750V, it is necessary to connect 300 capacitors each having a rated voltage of 2.5V in series. There are also cases in which some capacitors are connected in series and the others are connected in parallel to provide a necessary capacitance.
Capacitors, however, have variations in characteristics, and therefore, are applied with different voltages from each other. If charged without considering this, capacitors may be degraded, thereby shortening their life.
Under such circumstances, there have been proposed electricity accumulating devices in which a large number of capacitors have small variations in the degree of degradation, and hence, a long life.
FIG. 13 is a block circuit diagram of a conventional charging device. As shown in FIG. 13, the electricity accumulating device includes capacitors 501 connected in series, and balanced voltage adjusting portions 503 connected to capacitors 501 at their both ends. Each capacitor 501 is also connected at its both ends via two switches 507 to a corresponding sampling capacitor 505 for measuring the voltage across the corresponding capacitor 501. Balanced voltage adjusting portions 503 and switches 507 are connected to controller 509. Although it is not illustrated, capacitors 501 connected in series are also connected to the motor, the generator, the battery, the loads, and other components of the vehicle via charge-discharge circuits.
Each balanced voltage adjusting portion 503 includes a series circuit connected to both ends of the corresponding capacitor 501. The series circuit includes balance switch 511 and balance resistor 513. Each balanced voltage adjusting portion 503 also includes two partial pressure resistors 515 connected in series, which are also connected to both ends of the corresponding capacitor 501. The connection point of two partial pressure resistors 515 is connected to one input of comparator 517. The other input of comparator 517 is connected to digital potentiometer 519. Digital potentiometer 519 is connected to reference supply 521 and controller 509 so as to output a reference voltage in accordance with the instructions from controller 509. The output of comparator 517 is connected to balance switch 511 so as to control its on-off operation.
This electricity accumulating device operates as follows. First, controller 509 calculates the degree of degradation of each capacitor 501. More specifically, controller 509 calculates a capacitance C from the gradient in the change of the voltage across each capacitor 501 when it is charged at a constant current, and also calculates an internal resistance R from the change in the voltage across each capacitor 501 when the charge is suspended. Controller 509 then calculates the differences between the capacitance C and its predetermined degradation limit and between the internal resistance R and its predetermined degradation limit, thereby determining the degree of degradation from the differences. Therefore, the smaller the differences, the greater the degree of degradation.
Next, controller 509 calculates the average of the degree of degradation of all capacitors 501, and determines the balanced voltage of each capacitor 501 in such a manner as to reduce variations in the degree of degradation in all capacitors 501. In the case of a highly degraded capacitor 501, controller 509 determines a balanced voltage that reduces the voltage across the capacitor 501 so as to delay the degradation. Then, controller 509 controls each balanced voltage adjusting portion 503 to make the voltage across each capacitor 501 the balanced voltage.
Thus, the balanced voltages of capacitors 501 are adjusted so as to reduce variations in the degree of degradation in all capacitors 501. This delays the degradation of highly degraded capacitors 501, thereby allowing all capacitors to reach operating limits substantially at the same time. As a result, the electricity accumulating device has a long life. This technique is disclosed in Patent Literature 1.
The above-described conventional electricity accumulating device, however, is required to perform the following complex operations while capacitors 501 are being charged with a constant current. First, the capacitance C and the internal resistance R of each capacitor 501 are calculated. Next, the degree of degradation of each capacitor 501 is calculated from them. Then, the balanced voltage of each capacitor 501 is calculated from the average of the degree of degradation of all capacitors 501 in such a manner as to reduce variations in the degree of degradation in all capacitors 501.
Patent Literature 1: Japanese Patent Unexamined Publication No. 2007-124883